DocID026923 Rev 1

9/14

AN4587

Inpact of input current spikes on real applications

14

Table 1 summarizes the amplitude of the Iib spikes considering different input impedances

seen by the input pin of the TSZ121. These values are calculated from simulations.

We can observe that the higher the impedance on the input pin of the TSZ121, the lower the

Iib spikes. Actually, these current spikes follow an internal path with lower impedance.

This means that the spikes generated by a chopper op amp should not disturb a high

impedance sensor.

2.2

Filter input current spikes thanks to a common mode

capacitor

The TSZ121 should not necessarily be used with a high impedance sensor which naturally

smooths the Iib spikes.

In the case of a low impedance sensor, it may be interesting to filter the input current spikes

by using a common mode capacitor as shown in Figure 17.

Figure 15. Rin_load = 100 kΩ

Figure 16. Rin_load = 1 MΩ

Table 1. Iib spike amplitude vs. Rin_load

Rin_load

0 Ω

1 kΩ

10 kΩ

100 kΩ

1 MΩ

Amplitude Iib spike

1.15 μA

125 nA

13 nA

1.5 nA